Ultrasonic washing methods are known in which one ultrasonic vibrator is connected to one oscillator which produces an ultrasonic wave of a first frequency in a short time which is transmitted to washing water contained in a washing vessel. The ultrasonic vibrator is then switched to another oscillator and the ultrasonic wave produced of a second frequency is transmitted to the washing water in the next short time (see Japanese Patent Application No. 312620/88).
In this ultrasonic washing method, a plurality of air bubbles are generated in the washing water by the ultrasonic wave of the first frequency. Before these bubbles disappear, the ultrasonic wave of the second frequency is transmitted within a short time, and the remaining bubbles are destroyed by the ultrasonic wave of the second frequency. Also, the bubbles generated by the ultrasonic wave of the second frequency are destroyed by the next ultrasonic wave of the first frequency. Accordingly, the pressure of the ultrasonic wave for washing is increased because the pressure of the sound generated by the destruction of the bubbles is added to that of the ultrasonic waves of the first and second frequencies.
In the ultrasonic washing method, even if the ultrasonic wave of the second frequency is transmitted to the washing water before the bubbles generated by the ultrasonic wave of the first frequency disappear, the addition of the sound is not effective, because the greater part of the bubbles generated by the ultrasonic wave of the first frequency disappear.